#include <cstdio>
#include <algorithm>
using namespace std;
const int MAXN=5050;
int a[MAXN];
int p[MAXN];
int n,k;
int t;
int main(void){
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++){
        scanf("%d",&t);
        p[i]=p[i-1]+t;
    }
    double res=p[n]*1.0/n;
    for(int i=1;i+k-1<=n;i++){
        for(int j=i+k-1;j<=n;j++){
            //printf("%d %d %d\n",i,j,(p[j]-p[i-1])*1.0/j-i+1);
            double tmp=(p[j]-p[i-1])*1.0/(j-i+1);
            res=max(res,tmp);
        }
    }
    printf("%.15lf\n",res);
    return 0;
}
